The data rates that can be attained using electrical signals become more difficult to achieve as the signal path becomes longer and the data transmission rate becomes higher. One solution is to convert the transfer of signals from the transfer of electrons to the transfer of photons which are capable of carrying high speed, high data rate computing signals (hundreds of megahertz to gigahertz frequencies) over long distances with respect to those attainable using electronics. These interconnects may be chip-to-chip, module to module, board to board, part of a backplane, or box to box. Additionally, it can be desirable to use photons rather than electrons to maintain the integrity of the signal.
Optical data transfer can be accomplished by an optical waveguide having a light-carrying core material embedded in a cladding material. The optical signal is transmitted through the core material via total internal reflection. Optical waveguides are used at the printed circuit board level for clock distribution and interconnection of single chip packages. Optical waveguides can be used at the backplane, board, and MCM level to accomplish clock and signal distribution. On silicon or other substrates, optical waveguides can be used to accomplish chip-to-chip connections.
Another use of waveguide components is to provide special functions in larger systems. Devices such as couplers, splitters, and combiners are in general use throughout the fiber optics industry.
Useful optical waveguides must have low optical transmission loss, low optical absorbance, and controllable refractive index and birefringence.
Useful optical waveguide structures consist of a light-carrying core material and a cladding. One requirement for the waveguide structure is that the refractive index of the cladding material be less than the refractive index of the core material. For passive guides, the cladding could be air, but polymer claddings are typically preferred so that the core material is isolated from any conducting (metallization) layers. With polymeric core and cladding layers one also may have the capability of adjusting the refractive index to provide a specific refractive index ratio between the core and cladding materials.
A planar waveguide has two modes (TE and TM) for both core and cladding, and the refractive index of the core is greater than the refractive index of the cladding in both modes. All waveguides coated on a planar surface will have a TE and a TM mode. The TM mode (also referred to as n.sub..perp.) is perpendicular to the planar surface and the TE mode (also referred to as n.sub..parallel.) is parallel with the planar surface. The lightwave carried by the waveguide can be described by two components polarized in the TE and TM modes, or polarized perpendicular and parallel to the substrate surface. The lightwave is confined in a mode when the refractive index of the core is greater than the cladding (in that mode). "Confined" means light will not escape from the waveguide and "unconfined" means that the light will be dispersed.
Birefringence is a measure of the difference in index for the two polarizations, TE and TM, that exist in planar waveguides. Birefringence can arise from a number of causes, many of which may be related to the chemical structure of the polymer used in the waveguide. Two common causes of birefringence are the orientation of the polymer, and strain induced by the process of forming the waveguide. Generally, a large birefringence is detrimental to the performance of the waveguide as it causes the two polarizations to have different properties. The properties most affected by birefringence are the mode size and the propagation velocity. A difference in mode size reduces the efficiency of coupling the waveguide to fiber (which has a circular mode). A difference in propagation velocity leads to dispersion and places an upper bound on the rate at which data may be effectively transmitted through the device. Birefringence is measured as a continuous scale with 0.0 being the ideal. Values from about minus 0.05 to about plus 0.05 will provide an acceptable polyimide for use in a waveguide with a birefringence from minus 0.025 to plus 0.025 being preferred.
While it is desired to minimize birefringence of both core and cladding materials in planar waveguides, it is equally desirable to minimize the difference in birefringence between the core and cladding. The following discussion illustrates how differences in the birefringence between the core and cladding materials affect waveguide performance. When the core refractive index is greater than the cladding refractive index in either TE or TM mode, that mode will always be confined, and waveguiding will exist in that mode. It is desirable to have both TE and TM modes equally confined, or equally birefringent. However, several core/cladding refractive index relationships exist in birefringent media. Consider the case when TE is confined, that is, when the refractive index of the core is greater than the refractive index of the cladding in the TE mode. If the refractive index differences in TM and TE are equal, the waveguide mode, to first order, will be the same for both the TE and TM modes. This illustrates the most preferred case, when the difference in birefringence of the core and cladding materials is zero. If the refractive index difference in TM is greater than in TE and the TM refractive index for the core is greater than for the cladding, the TM mode will be more tightly confined than the TE mode resulting in a smaller mode diameter for the TM mode. If the refractive index difference in TM is less than in TE and the TM index for the core is greater than for the cladding, the TM mode will be less confined than the TE mode resulting in a larger mode diameter for the TM mode. If the TM core refractive index is less than the TM cladding refractive index, there is no confined TM mode and significant amounts of optical power can be radiated away from the waveguide. Therefore, the ability to control both the refractive index and the birefringence of the core and the cladding are particularly important in planar optical waveguides. The same relationships will be true if TE and TM are transposed in the above discussions; i.e., if the TM mode is confined and the TE indices are considered.
For optical fiber-planar waveguide coupling applications it is important that the refractive index and the birefringence of the core and cladding materials of the planar waveguide are controllable and that the birefringence of the core and the cladding are closely matched to reduce losses caused by connecting a planar waveguide to fiber, or by scattering.
A desirable property in an optical waveguide is high thermal stability, which is necessary so that the waveguide will survive electronic packaging and assembly processes used in manufacturing. The optical multichip module would have to survive semiconductor assembly processes such as die attach, metallization, and wire bonding. The printed circuit board would have to survive reflow soldering and rework. The optical coupler would have to survive fiber attach and assembly operations.
The ability to wet-etch the core or cladding of an optical waveguide into channels or ridges having smooth sidewalls is also a desirable property of an optical waveguide, as is the ability to form multi-layer structures by overcoating the polyimide layer with another without the first layer being affected by the solvent used in the overcoat layer.
Polyimides are known to have the thermal stability required for electronic and semiconductor applications; however, many semiconductor grade polyimides display a high optical absorbance in the near IR visible range. Since typical commercial laser and light sources emit in the near IR visible range (350 nm to 2,000 nm) a polymer having a high optical absorbance in this region is generally not desirable for use as an optical waveguide. U.S. Pat. No. 5,304,626 discloses copolyimides of BPDA, including those incorporating 6FDA and APBP, CODA, ODA, BDAF, BAAF, and FAPB, having improved solvent resistance characteristics. The only diamines specifically exemplified are APBP and CODA. The reference does not suggest the use of the copolymers as optical waveguiding materials or disclose any optical properties for the polyimides.
U.S. Pat. No. 5,317,082 discloses photodefinable photosensitive copolyimides and waveguide structures thereof having a 6FDA/BTDA dianhydride component, an aromatic diamine component having bulky methyl groups ortho to the amine, and a fluorinated co-diamine component to reduce birefringence. As the co-diamine is incorporated in the copolymer structure, the change in birefringence is greater than the change in refractive index which makes these polymers unsuitable as core/cladding pairs for many applications. Additionally, the photosensitive copolyimides have relatively higher birefringence, making them less suitable for optical waveguide applications than the polyimides of the present invention.
U.S. Pat. No. 5,108,201 discloses polyimides and copolyimides which incorporate 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFMB) and their use as optical waveguides having a controllable core/cladding refractive index ratio. Examples are given which demonstrate the use of 6FDA/PMDA/TFMB copolymers to control the refractive index between 1.55 and 1.65. However, there are no specific examples of 6FDA/BPDA/TFMB polyimides, and there is no consideration given to matching the birefringence of the core/cladding pair or to how that would be achieved by selecting appropriate copolymer compositions.
U.S. Pat. No. 5,344,916 discloses polyimide and copolyimide films having negative birefringence for use in liquid crystal displays, and a method for controlling the negative birefringence. Copolymers of BPDA/PMDA/TFMB are used to illustrate the ability to change the negative birefringence by varying the polymer dianhydride composition. Increasing the amount of PMDA in the dianhydride component increases the negative birefringence. However, the change in birefringence is greater than the change in refractive index, which would be undesirable for a planar optical waveguide core/cladding application. There is no suggestion of the use of these polymers as optical waveguides.
A. J. Beuhler, et al.; "Fabrication of Low Loss Polyimide Optical Waveguides Using Thin-Film Multichip Module Process Technology," IEEE Transactions on Components, Packaging, and Manufacturing Technology--Part B, Vol. 18, No. 2, May 1995 discloses a wet chemical patterning process for the fabrication of low loss waveguides using photosensitive polyimides of 6FDA and BAAF which are rendered photoimageable by co-polymerizing low concentrations of a photosensitizer and alkylated photocrosslinking group into the polymer backbone. The refractive index of the polyimides can be varied for core/cladding pairs by replacing aliphatic hydrogen atoms in the polyimide with fluorine atoms by varying the ratio of methylated to fiuoromethylated diamines in the reaction mixture.
T. C. Kowalczyk, et al., "Loss Mechanisms in Polyimide Waveguides", J. Appl. Phys. 76(4):2505 (15 Aug. 1994) discloses waveguide losses in thin film polyimides as a function of cure cycle and structure in polyimides of 6FDA/APBP, 6FDA/BDAF, and 6FDA/BAAF/R (R=alkylated photocross-linking group). Increased fluorination did not have a predictable effect on birefringence. In the polymides having no R group increased fluorination was reported to decrease both the refractive index and the birefringence, however, the 6FDA/BAAF/R polyimide had lower refractive index but higher birefringence than the non-alkylated polyimides.
A. J. Beuhler, et al, "Optical Polyimides for Single Mode Waveguides", SPIE, Vol. 1849, Optoelectronic Interconnects, pp. 92-103 (1993) discloses the synthesis and optical characterization of fluorinated polyimide systems with potential use in passive waveguides and electro-optic devices and reviews the effect of fluorination on optical properties such as refractive index, birefringence, and near-infrared absorbance in terms of optical performance requirements. Synthetic methods of tuning the parameters and refractive index in order to achieve appropriate core/cladding differentials is discussed. The polyimides disclosed are BPDA/pPDA; BPDA/APBP; BPDA/FAPB; 6FDA/APBP; and 6FDA/BDAF. The fluorinated polyimides have the disadvantages of poor solvent resistance and difficult fabrication. Photochemical crosslinking was used to increase solvent resistance and introduce patternability into fluorinated polyimides. At paragraph 3 of the Introduction it is stated that rigid rod polyimides, such as those based on biphenyldianhydride, are anisotropic and also tend to be highly colored due to intramolecular charge transfer complexes and that these properties lead to substantial optical, absorption and scattering losses.
T. C. Kowakczyk, et al., "Guest-Host Crosslinked Polyimides," J. Appl. Phys. 78 (10):5876 (15 Nov. 1995) discloses waveguides in which 6FDA/BAAF/R is used as a cladding with a 6FDA/BAAF/R core doped with DADC or DCM and illustrates how the refractive index of the core material is raised by incorporating an NLO dopant. The dopant may cause losses in the form of scattering sites and absorption tails.
T. C. Kowalczyk, et al., "Guest-Host Crosslinked Polyimides for Integrated Optics," ACS Symp. Ser. (1995), 601 (Polymers for Second-Order Non-Linear Optics) p.381-400 reports poling issues related to multi-layer films which incorporate 6FDA/BAAF/R and also points out a problem of increased optical loss due to long absorption tails in the chromophore.
See also Beuhler, et al., "Optical Waveguides from Photosensitive Polyimides", Extended Abstracts, 5th International Conference Polyimides, Nov. 2-4, 1994.
Meinhardt, et al., "Characterization of Thermally Stable Dye-Doped Polyimide Based Electrooptic Materials," Mater. Res. Soc. Symp. Proc. (1994), 328 (Electrical, Optical, And Magnetic Properties Of Organic Solid State Materials), p. 467-475. 6FDA/BAAF/R and 6FDA/TFMB/R, where R is an alkylated aromatic crosslinking species whose structure is not disclosed, were doped with oxazoles and the absorptive losses were measured. Thermal decomposition of certain oxazole-doped polyimides were greater than for the undoped polyimides.
Meinhardt, et al., "Characterization of Crosslinked Electrooptic Polyimides," Proc. SPIE-Int. Soc. Opt. Eng. (1994), 2143 (Organic, Metallo-Organic and Polymeric Materials for Nonlinear Optical Applications) p. 110-16, discusses the electro-optical properties of doped 6FDA/BAAF/R polyimides, doped with DCM and DADC, a bis (carbazole) analog of DCM and two oxazoles. Absorption losses with the different dopants are reported.
P. A. Cahill, et al., "Polyimide-based Electrooptic Materials", Nonlinear Optical Properties Of Organic Materials VI, SPIE Vol. 2025, pp. 48-55 (1993). The effects of doping 6FDA/BAAF/R and 6FDA/TFMB/R polyimides (R=alkylated photocrosslinking group) on the dielectric constant, refractive index and coefficient of thermal expansion of the polyimides are presented. Azole dyes were used as dopants.
Beuhler, et al., "Polyimide Optical Waveguides," Organic Thin Films For Photonic Applications, 1993 Technical Digest Series, Vol. 17, p. 254-257. The paper discusses the interest in making planar waveguides from polyimides and indicates that most aromatic polyimide compositions that exhibit low optical loss are based on 6FDA. It also states that rigid rod polyimides such as those based on biphenyl dianhydride exhibit outstanding thermal-mechanical properties such as low thermal expansion and high glass transition temperature making them attractive for semiconductor coating applications, but that many of the rigid rod polyimides have the disadvantages of being highly anisotropic and intrinsically colored due to intramolecular charge-transfer complexes that form between the electron rich diamine and electron deficient dianhydride which leads to substantial optical loss. The effect of polyimide structure on birefringence is shown in FIG. 1 for BPDA/pPDA; BPDA/APBP; BPDA/FAPB; 6FDA/APBP; and 6FDA/BDAF and 6FDA/BAAF/R. In paragraph 2 on page 2 it states that 6FDA/BDAF and 6FDA/BAAF/R have the highest transparency in the near infrared visible and should produce the lowest attenuation waveguides provided that scattering losses are low, but they may suffer from poor solvent resistance and lower thermomechanical properties. It states that in these cases, co-polymerization with rigid monomers can be used to improve thermomechanical properties. This is repeated in the last sentence of the article; however, there are no examples of making and testing such polyimides. C. Feger, et al., "Polyimide Waveguiding at 830 nm," Ann. Tech. Soc. Conf. Soc. Plast. Eng. 49th, p. 1594-1597 (1991) describes waveguiding properties of some polyimides which contain one or two hexafluoroisopropylidene groups in the backbone, including PMDA-6FDAm, BTDA-6FDAm, and BPDA-6FDAm (6FDAm is the diamine referred to herein as BAAF). The behavior of the optical losses such as scattering losses caused by birefringence of the polyimides at different wavelengths and temperatures was measured.
NASA Technical Support Package LAR-13539 and U.S. Pat. Nos. 4,595,548 and 4,603,061 disclose transparent aromatic polyimides derived from various dianhydrides bridged by a flexible "separator" moiety, including 6FDA, ODPA and BDSDA, and ether or thioether bridged diamines; the use of BPDA or other unbridged dianhydrides or diamines is not disclosed. The polyimides were evaluated as second-surface mirror coatings on thermal control systems.
Stoakley, et. al., J. AppL Polym. Sci., Vol. 51, 1479-83 (1994), report copolymers of fluorinated dianhydrides and BPDA with a fluorinated diamine, DABTF which were prepared as films and composite laminates for use as aircraft matrix resins. The copolymers were found to have decreased solubility and excellent optical transparency, although incorporation of BPDA increased the UV cutoff and decreased the percent transmission slightly. There is no suggestion of using the copolyimides for waveguiding.
U.S. Pat. No. 4,952,669 discloses copolyimides and copolyamic acids incorporating 2-(3-aminophenyl)-2-(4-aminophenyl)hexafluoropropane (3,4'-6F diamine) having improved solubility characteristics, low dielectric constants and improved thermal flow properties as a consequence of the metalpara positioning of the amino groups on the diamine including BPDA/6FDN3,4'-6F Diamine. The reference does not disclose or suggest the use of the copolymers as optical waveguiding materials.
U.S. Pat. No. 4,978,738 discloses high molecular weight (&gt;90,000) polyimides of BAAF diamines and BPDA, ODPA, and BTDA dianhydrides which optionally contain a suitable amount of 6FDA, but it does not suggest their use as waveguiding materials or how polymer compositions having useful properties in waveguiding structures would be selected.
U.S. Pat. No. 4,954,609 discloses a process for producing polyamic acids and polyimides having a controllable molecular weight and molecular weight distribution wherein at least one of the diamine or dianhydride contains a fluorinated bridging group. There is no suggestion of possible use of the polyimides as waveguides or in waveguide structures.
U.S. Pat. No. 5,025,089 discloses copolyimides of BAAF and at least one other diamine with pyromellitic dianhydride (PMDA) and at least one other dianhydride having a diaryl nucleus which are soluble in organic solvents such as methyl ethyl ketone and N-methyl pyrrolidone (NMP) and which display improved mechanical and electrical properties. There is no suggestion of using these polyimides as optical waveguides. Furthermore, the use of PMDA in a copolyimide will cause an undesirable change in birefringence making these copolymers unsuitable for optical waveguide structures.
U.S. Pat. No. 5,049,649 discloses high molecular weight colorless optically clear films consisting essentially of BAAF or its meta-isomer and 6FDA. There is no consideration given to forming waveguides or waveguide structures, or to adjusting refractive index and birefringence by forming suitable copolymers.
U.S. Pat. No. 5,089,593 discloses polyimides useful in microelectronic devices which incorporate 4,4'-bis(4-amino-2-(trifiuoromethyl)phenoxybiphenyl. The reference does not disclose any optical properties of these polyimides or suggest the use of these polyimides as optical waveguiding materials.
U.S. Pat. No. 5,354,839 discloses polyimides incorporating fluorinated bis(3-aminophenoxy)arene moieties. An example of a 6FDA/BPDA copolymer is given, however, there is no suggestion of its use as a waveguiding material or that it would form a useful component of a core/cladding pair of waveguiding materials.
U.S. Pat. No. 5,326,600 discloses polyimides incorporating a substituted diaminobiphenyl moiety and their use as an alignment film in liquid crystal devices. There is no suggestion that it would be advantageous to form a copolymer or any suggestions that the polymers disclosed might have utility as an optical waveguide or as a component of a core/cladding pair of waveguiding materials.
The present invention provides thermally stable, low optical loss, low optical absorbance, polyimide waveguides.
The polyimides of the present invention provide 6FDA/DIAMINE, BPDA/DIAMINE, and 6FDNBPDA/DIAMINE polyimide optical waveguide core/cladding polymer pairs in which both the refractive index and birefringences of the core and cladding can be controlled. Birefringence is a measure of the optical anisotropy (i.e., wherein the properties are different in one direction, for example in-plane, than they are in another direction, for example, out-of-plane). The typical rigid rod polyimides that are generally preferred for semi-conductor applications are highly oriented in the plane of the coating and thus exhibit a high birefringence. High birefringence measurements correspond to high optical scattering losses and irreproducibility of the refractive index and are undesirable for optical waveguides (for which the ideal birefringence is 0.0). Thus, it is desirable to select polyimides having relatively low birefringence, preferably of less than about 0.025, for planar waveguide applications.
It is also desirable to be able to adjust the refractive indices of the core and cladding to contain the optical signal without substantially changing the birefringence and increasing light scattering. The refractive index of the polyimides of the present invention can be adjusted by substituting BPDA for 6FDA in the polyimide composition. This provides core-cladding pairs in which refractive indices of the core and cladding are adjusted and birefringences of the core and cladding are controlled to provide the desired waveguiding conditions.
It is surprising and unexpected that the refractive index and birefringence of a 6FDA/BPDA/DIAMINE optical waveguide core or cladding could be controlled by replacing 6FDA with BPDA in the core and cladding so that the change in the birefringences of the core and cladding polyimides is less than the difference in the respective TM refractive indices of the core and cladding. Polyimides containing BPDA generally are thought to have higher birefringence. Thus, it is unexpected that substituting BPDA for 6FDA in the polyimides does not produce birefringence levels that would be undesirable for waveguide uses.
The BPDA-containing polyimides of the present invention are solvent resistant and can be fabricated into multi-layer structures by overcoating one polyimide layer over another which provides an advantage over solvent sensitive polyimides which may be affected by the solvent used to overlay a layer of polyimide or other material.
It is an object of this invention to provide a core/cladding pair of materials forming a waveguide which have closely matched physical and mechanical properties. Typical waveguide operating conditions may involve significant departures from ambient conditions in demanding applications where temperature and humidity vary widely. In order for the waveguide to have optimum performance under a variety of operating conditions, it is necessary that the glass transition temperature, moisture absorption, coefficient of thermal expansion, and modulus of the core and cladding materials be closely matched. While it is known to change the refractive index by forming copolymers, this has not been demonstrated with retention of physical and mechanical properties. The present invention provides core/cladding pairs of materials having closely matched physical and mechanical properties.